In the field of optical communications, a communication system combining a synchronous detection method which dramatically improves frequency usage efficiency and signal processing is attracting attention. Compared to a system constructed with direct detection, it is known to be capable of compensating waveform distortion of transmitting signals due to wavelength dispersion and polarization mode dispersion received owing to optical fiber transmission by receiving as digital signals in addition to improving receiving sensitivity. Therefore, introducing as an optical communication technology of the following generation is considered.
A digital coherent method represented by Non-Patent Literatures 1 and 2 adopts a method to compensate quasi-static wavelength dispersion with a fixed digital filter (e.g., a tap number of 2048 taps with a dispersion of 20000 ps/nm against a signal of 28 Gbaud) and to compensate polarization mode dispersion having variation with an adaptive filtering with the small tap number (e.g., about 10 to 12 taps for polarization mode dispersion of 50 ps) using blind algorithm.